Paralleling 18650's is relatively easy. You need to match voltage to within a few mV and make sure the connection is really solid (welded) to ensure they stay paired perfectly. Flaky connections, putting cells in series, impact damage, bad chargers etc are the risky bits, a solidly connected pair of 18650's is to a close approximation just as safe as a single cell, but it does have twice the short circuit current so you are going to have to be more careful around them. But at least the casings will be at the same potential.
I've built a 17P10S pack which was a pretty interesting (and scary) effort but it has been working flawlessly for years now with just one inspection of the guts after two years to make sure that nothing was coming loose (it's on an s-pedelec e-bike). In a big pack like that it's the spaces between the alternating blocks of cells and on top where the interconnects are that the real risk lies, besides the fact that the short circuit current of that pack is just shy of a kilo ampere so you really don't want to drop a tool or a piece of interconnect strip on that.
> Paralleling 18650's is relatively easy. You need to match voltage to within a few mV and make sure the connection is really solid (welded) to ensure they stay paired perfectly.
These requirements are already not easy, and there are still plenty of things to consider for using LiPos in parallel (e.g. identical health, preferably batteries from same batch, to increase chance they age identically)
OP exaggerates the difficulty. You can trivially parallel the vast majority of lithium batteries so long as their voltage is reasonably close (I personally wouldn't fuss much over a 100mv difference, or even more in most cases, unless it's a massive battery or a power cell capable of delivering and accepting very high currents - charging most cells will often involve raising the voltage 200-300mv during the constant current phase, so you can safely parallel with a difference like that)
You can match up pretty different batteries in parallel as well. One will take more load etc, but this is not usually a problem. It's not ideal, but I think people often exaggerate the dangers.
Series is much more problematic, since most balancing circuits have very limited capacity to balance mismatched batteries.
> These requirements are already not easy,
Get them reasonably close, then leave then connected by a moderate value resistor for hours. Then you're within mV.
If you're just concerned with getting 95% capacity from each battery, "close" is good enough for the rest.